Well, only one of my bolts would come out, even after spray PB on them for days. The passenger side one behind/under the seat was the biggest pain, and I couldn't figure out how to get my sawzall in there.
First I had to get the passenger seat out of the way. Then I ground down the bolt head.
Then, I jacked up the body, using a cut 2x4 and a floor jack. As the body went higher, I put some 2x4 pieces between the body and the frame, just in case the jack failed.
Underneath, the top bushing came loose, but I needed to jack the body up more to get it over the bolt.
Once that was removed, I cut the bolt with the sawzall, as close to the "box" as possible. because the holes in the frame are fairly big, you can maneuver the the rest of the bolt and it's attached pieces out of the frame box. You might have to beat on it. I used a chisel and a hammer from the back, and it quickly popped out the front.
When installing the new mounts, be sure to put tape over the hole in the frame that is inside this "box."
Does that help?